The invention relates to hot-melt resins based on unsaturated polyesters and to the use of these hot-melt resins as casting and impregnating compositions or coating materials.
Casting and impregnating compositions are hot-melt resins which are employed in particular in electrical engineering in accordance with techniques which are general knowledge, such as dipping, hot-dipping, dip-rolling, flooding, casting, vacuum impregnation, vacuum pressure impregnation or trickling for impregnating windings of electronic components.
The hot-melt resins are also suitable for impregnating absorbent materials such as carrier materials for sheetlike insulating materials, especially glass silk or mica tapes. In such applications, curing may be interrupted at the B stage, to give curable prepregs.
Moreover, the hot-melt resins may be used as coating materials for primed or unprimed substrates such as vehicle body parts, industrial components or packaging, especially metal and/or plastics substrates.
Not least, the hot-melt resins may be used to produce an abrasion protection for high-speed machine parts.
Mainly, however, the hot-melt resins are used as casting and impregnating compositions for electrical and electronic components.
The majority of known casting and impregnating compositions for electrical components such as motor windings or transformer windings are liquid formulations based on unsaturated polyesters, which are in solution in compounds containing vinylic and/or allylic unsaturation, such as styrene, (alpha-methylstyrene, vinyltoluene, allyl phthalates and monomeric or oligomeric acrylic or vinyl esters. Also known, furthermore, are liquid compositions based on dicyclopentadienyl(DCPD)-modified unsaturated polyester resins, which can be cured without additional liquid and/or solid comonomers.
A disadvantage of these known casting and impregnating compositions is the use of the abovementioned monomers, which are vital for establishing the required, relatively low processing viscosities and for the curing of the unsaturated polyester resins. These monomers are injurious to health and irritant to skin. In the case of known applications of casting and impregnating compositions comprising these substances, moreover, there are mass losses of from 20 to 30%. These considerable amounts must be removed from the workplace in order to avoid health hazards for the employees. The evaporated amounts, drawn off under suction, are generally disposed of in waste-air incinerators. This gives rise to unwanted emissions. Moreover, the incineration of such quantities of material is uneconomic. Furthermore, there is a risk of these monomers not being incorporated completely in the course of curing. Residual monomers in the cured compositions may emerge, especially from electrical insulating compositions, which tend to become warm during use, and may lead to odor nuisance and health hazards. They may also gradually aftercure within the molding compounds, so undesirably altering the service properties of the said compounds.
A considerable technical problem with all liquid casting and impregnating compositions, including those curable without monomers, is the deficient storage stability of the formulations made ready for use by the addition of catalysts. In transit, especially at summer temperatures, there is always the risk of unwanted premature reactions, ranging from partial gelling to complete reaction, which is dangerous.
For these reasons, there have already been a number of attempts to use solid resins as casting and impregnating compositions. Using such hot-melt resins, it ought to be possible to reduce markedly the emissions and to improve the storage stability.
European Patent EP-A-0 101 585 proposes casting and impregnating compositions of high thermal stability and heat distortion resistance which comprise unsaturated polyesters modified with dicyclopentadiene and/or other 1,3-diolefins and as their alcohol component comprise N-hydroxyalkylimides of mono-unsaturated, unsubstituted or substituted, cyclo-aliphatic 1,2-dicarboxylic acids. The resins are processed as solutions in styrene, for example.
European Patent EP-A-0 118 786 discloses casting and impregnating compositions comprising unsaturated polyester resins which may have been modified with dicyclopentadiene, which contain cyclopentene or cyclohexene groups, and which are prepared in a complex process conducted preferably in two stages. Curing is effected at temperatures above 200° C. in the presence of free-radical initiators which decompose above 140° C., or by irradiation with electron beams. The casting and impregnating compositions are processed as solutions in styrene, for example.
European Patent EP-A-0 260 688 avoids the disadvantages of the high curing temperatures of EP-A-0 118 786, of more than 200° C., by means of a modified two-stage process in which the high temperatures of the aftercure in the second stage are reduced to 120–200° C. through the use of a specific selection of free-radical initiators.
Japanese laid-open specification JP-A-53 05 97 91 describes polyesterimide resins which are prepared using at least tribasic carboxylic acids, polyester polyols and amines. These substances are used as thermoplastic hot-melt resins for electrical insulation.
German Patents DE-A-26 48 351 and DE-A-26 48 352 relate to injection-moldable compositions which comprise unsaturated polyester resins, peroxides and fillers and are suitable for use as hot-melt resins for electrical insulation.
German Patent DE-A-16 40 428 describes the use of component-specific, ring-shaped elements made from unsaturated polyester resins, which to improve the blocking resistance are coated with waxes. These rings are placed on the winding head of the components and melted by heating, after which the resin, in the amount controlled by the ring size, flows into the components, where it cures thermally. In this process, the use of wax is required in order to ensure that the resins, which are not blocking-resistant, are storable, despite the fact that the waxes disrupt the process.
The solution proposed by German Patent DE-A-195 42 564 to the problem is to use free-radically curable resin systems comprising unsaturated polyester resins, appropriate curing agents (catalysts), if desired, comonomers and/or ethylenically unsaturated polymers such as diallyl phthalate oligomers, with or without further customary additives.
Despite the advantages of the hot-melt resins in their use as casting and impregnating compositions, and the abovementioned efforts at developing appropriate materials, no applications have been disclosed to date.
A particular problem which has been found for application is the incorporation of the free-radical initiators (e.g., peroxides) into the melts at relatively high temperatures during the formulation of the hot-melt resins. Moreover, it is difficult to incorporate the hot-melt resins into the components by melting without initiating a crosslinking reaction. With the hot-melt resins proposed to date, the stability window between processing temperature and reaction temperature is too small for reliable handling.